ASTRONOMY & ASTROPHYSICS MAY I 1999, PAGE 429 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 136, 429–444 (1999) A search for spectroscopic binaries among Herbig Ae/Be stars?,?? P. Corporon and A.-M. Lagrange Laboratoire d’Astrophysique de l’Observatoire de Grenoble, BP. 53, F–38 041 Grenoble Cedex, France Received March 4; accepted November 17, 1998 Abstract. We present the results of a spectroscopic sur- binaries (see a detailed review in Clarke 1996), but obser- vey of binaries among 42 bright (mV < 11) Herbig vations are needed to constrain further these models. Ae/Be stars in both hemispheres. Radial velocity vari- Secondly, a fundamental role of binary studies is to ations were found in 7 targets, 4 are new spectroscopic allow the direct determination of physical parameters. binaries. The Li i 6 708 A˚ absorption line (absent feature Noticeably, the stellar mass is only accessible through in simple HAeBe stars spectra) indicates the presence of the observation of gravitationally bound pairs of stars, by a cooler companion in 6 HAeBe spectrum binaries, 4 of straight application of gravitational law. which are new detections. Few stars classified as possible Main-Sequence (MS) binary stars are overall quite well Herbig Ae/Be stars are not confirmed as such. studied. However, as orbits of binary systems evolve with While for short-period (P<100 days) spectroscopic time, it is mandatory to derive the properties of the sys- binaries, the observed binary frequency is 10%, the true tems during the pre-Main Sequence (PMS) phase. A major spectroscopic binary frequency for Herbig Ae/Be stars issue is to quantify the binary frequency fb (the probabil- maybeashighas35%. ity that a given object is multiple, Reipurth & Zinnecker 1993) for young multiple objects, their separation distri- Key words: binaries: spectroscopic — stars: early-type — bution as well as their mass ratio. stars: pre-Main Sequence — stars: statistics Recent studies have shown that more than half of T Tauri stars, young stars having a mass M<1.5M , are members of a binary or multiple system (Mathieu 1992; Leinert et al. 1993; Reipurth & Zinnecker 1993; Prosser et al. 1994; Simon et al. 1995; Ghez et al. 1993, 1. Introduction 1997; Brandner et al. 1996; Padgett et al. 1997). Whether there is an overabundance of low-mass PMS binaries ver- 1.1. Multiplicity and evolution sus MS binaries is still a matter of debate, due to the difficulties to compare both statistics obtained with differ- Multiplicity is a major issue in stellar astrophysics. Firstly, ent approaches. Not only the employed techniques are dif- binary stars are very common among Main Sequence (MS) ferent, (MS stars were mainly spectroscopically searched stars: half of the MS field stars are known to belong to for, while PMS binaries were searched with high angular multiple systems (see Garmany et al 1980 for O type resolution imaging), but also the wavelength domains are stars; Abt et al. 1990 for B; Nordstr¨om et al. 1997 for different (optical observations predominate for MS stars, F; Duquennoy & Mayor 1991 for G; Mayor et al. 1992 for while infrared (IR) surveys of the young objects, mostly K; Leinert et al. 1997 for M). Thus, any stellar forma- embedded in dark regions, were used ipso facto). The only tion theory must explain this large abundance of multiple systematic effort aimed at finding new low-mass pre-Main systems. Various mechanisms have been proposed to form Sequence binaries with visible spectroscopy dates back from Mathieu (1992). Send offprint requests to:P.Corporon ? Based on observations collected at the European Southern The question is then whether the stellar density may Observatory (ESO), La Silla, Chile and at the Observatoire de have an effect on the formation of multiple systems. MS Haute–Provence (OHP), Saint–Michel l’Observatoire, France. stars are thought to have been formed in OB associa- ?? Table 1 only available in electronic form at CDS via tions or in dense clusters (Miller & Scalo 1978; Lada anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via et al. 1991), while PMS stars in T associations are formed http://cdsweb.u-strasbg.fr/Abstract.html in lower density environments which might enhance the 430 P. Corporon and A.-M. Lagrange: A search for HAeBe spectroscopic binaries production of binaries. However, Brandner et al. (1996) 1.3. Aim of the present paper observed T Tauri stars in OB and T associations and concluded that the binary frequency is the same for In this paper, we report the first results of a systematic MS and PMS low mass stars in the range of separation high resolution spectroscopic search for HAeBe binaries. 120 − 1 800 AU (except for the Taurus–Auriga star form- It is part of an extensive survey made on Herbig Ae/Be ing region). Moreover, Padgett et al. (1997) using HST stars, the other facet being high angular resolution imag- observations of PMS stars in dense clusters recently found ing of IR companions of HAeBe stars using Adaptative a comparable binary frequency for dense clusters and low- Optics systems (Bouvier et al. 1998; Corporon 1998). The density star-forming regions. They thus claim that multi- two approaches are complementary: while the former gives plicity is not influenced by the local stellar density (at least access to the study of short orbital periods (P = few hours in regions with densities ranging between 40 and 5 000 to few months) of double stars, the latter covers the do- stars pc−3). main of longer periods (P = many years). Since the pioneering work of Mathieu (1992), selec- The present paper is structured as follows: in Sect. 2, tive mass determinations for some low-mass PMS binaries we describe our sample, the instruments used as well as have been obtained (Padgett & Stapelfeldt 1994; Welty our observing strategy. We present in Sect. 3 the spectra 1995; Figueiredo 1997), but the sample of young multiple of some known and new spectroscopic HAeBe binaries. systems with orbital and physical parameters determined Notes on individual sources are given in Sect. 4 and in must be enlarged in order to test the early stages of stellar Sect. 5 we discuss our results. evolutionary models. 2. Observations of the HAeBe binaries 1.2. Multiplicity of intermediate-mass PMS stars 2.1. The sample As pointed out by Hillebrand (1994), the intermediate- mass PMS stars, namely Herbig Ae/Be (HAeBe) stars The observed Herbig Ae/Be stars were extracted from are found in various environments: in dense star form- Table 1 of Th´e et al. (1994) catalogue. We obtained high ing regions (containing few tens of HAeBe plus a myriad resolution spectra for 42 objects with mV < 11, consist- of T Tauri stars), in lower density groups of young stel- ing of our principal HAeBe sample. This sample represents lar objects (2 to 5 HAeBe stars sharing the same birth about 70% of the HAeBe candidates with mV < 11 from place) and in isolated molecular cores (where a central Table 1 of Th´e et al. (1994) catalogue. Most of the remain- HAeBe star and embedded young lower mass stars are ing stars were not observed either because they were al- found). Studying the binary frequency among HAeBe ob- ready well studied or because they are strong photometric jects may help to better understand the relation between variables and out of our limit of our observing capabilities the direct environment and the multiplicity status of the at their minimum brightness. stars during their earlier formation stages. A sub-group of other HAeBe candidates listed in Besides reinforcing the binary frequency estimate, Tables 2 to 5 from the catalogue of Th´e et al. (1994) were HAeBe binaries study is of great interest because direct also studied (sample T2–T5). They were included in our mass determination are fervently required to test the survey because they could be observed in parallel to our stellar evolution models for young intermediate-mass main program. Those stars were observed not only to test stars. the presence of a companion, but also to precise, when To date, however, the binarity status of Ae/Be Herbig possible, their spectral type or to test their belonging to stars has been far less surveyed, probably because these the Herbig Ae/Be stellar class. stars form a class less homogeneous than T Tauri stars We address in this paragraph the possible bias by the (Th´e et al. 1994). Few recent studies using infrared imag- so-called Branch effect. Branch (1976) pointed out that a ing (Li et al. 1994; Leinert et al. 1997b; Pirzkal et al. 1997) magnitude-limited sample favors the detection of double- have found a binary frequency (although based on limited lined spectroscopic binary (SB2). In other words, some samples) in excess by a factor 2 versus A/B type MS stars, binary stars could have been selected because their total (by considering G type MS stars degree of multiplicity fb, magnitude is below the magnitude limit, whereas indi- spectroscopically determined by Duquennoy and Mayor vidual stars are fainter than the mV limit. Such systems 1991 identical through the Main Sequence, as explained should be removed when establishing the binary frequency by Leinert et al. 1993). of the sample. However, in our case with high mass pri- Up to now, the pilot study made on the eclipsing and maries, unless the mass ratio is close to unity, the sec- spectroscopic triple system TY CrA (Lagrange et al. 1993; ondary component of a spectroscopic binary system con- Corporon et al. 1994, 1996; Beust et al. 1997, see also tribute to few percents of the combined flux.